So how do you measure "the best possible estimate of ability to pay the loan back"? Because I think that broad assumption is a lot more complex and interesting than a simple FICO score.
For a crude example, pick health history. The leading cause of bankruptcy in the US is medical bills due to a health crisis. This of course spills over into mortgage foreclosure. But someone might have excellent credit, and a high cancer risk. They get cancer, byebye loan. Are you accounting for that?
Someone has excellent credit, but works in a dying industry and has no skills to move to an equal-paying job in another industry. Their job goes away in five years, byebye mortgage. I can go on with these. But my point is, I don't know if you realize just what a difficult point you're making with your simple answer.